Currently, LTE (long term evolution) has been under development as a next generation high speed radio communication mode (or next generation high speed radio communication system). LTE is a communication mode specializing in packet communication. LTE-capable radio communication terminal devices presumably utilize a conventional radio voice communication system such as W-CDMA and cdma 2000 1x for voice communication by circuit switching. Here, W-CDMA and cdma 2000 1x are radio communication systems supporting voice communication.
Here, if a radio communication terminal device is always on standby both for a radio voice communication system and for an LTE system, the power consumption during standby is increased. However, if a radio communication terminal device is always on standby only for an LTE system, it is normally disabled from receiving incoming voice calls and fails to function as a so-called “telephone.” On the other hand, if a radio communication terminal device is always on standby only for a radio voice communication system, advantageous normally-on IP connectivity of LTE is wasted.
In order to resolve the above problem, LTE systems utilize a function called CS (circuit switched) Fallback. In the CS Fallback, the following procedures are executed. First, a notice of incoming voice call from a radio voice communication system is tunneled into the LTE system and transferred to the radio communication terminal device. Receiving the notice of incoming voice call, the radio communication terminal device suspends the LTE system and catches the radio voice communication system. Furthermore, the radio communication terminal device establishes line connection to the caught radio voice communication system and performs voice communication. After the voice communication is finished, the radio communication terminal device disconnects the radio voice communication system. Furthermore, the radio communication terminal device resumes the LTE system and returns to standby state in the LTE system. Similar procedures to the incoming voice processing are executed for outgoing voice processing in the CS Fall hack.
The CS Fallback reduces the power consumption compared with the case which is always on standby both for a radio voice communication system and for an LTE system. The CS Fallback is disclosed, for example, in Unexamined Japanese Patent Application KOKAI Publication No. 2009-267996 and 3GPP TS23.272 V8.3.0: Circuit Switched Fallback in Evolved Packet System; Stage 2.
However, when a radio communication terminal device suspends an LET system and catches a radio voice communication system, message transmission from the radio communication terminal device to the caught radio voice communication system may sometimes be inhibited by the access control of the caught radio voice communication system. In such a case, the radio communication terminal device wastes its power for executing suspension procedure and resumption procedure of the LTE system in spite of the fact that the voice communication is unavailable. Furthermore, in such a case, radio transmission/reception of control messages for the suspension procedure and resumption procedure lead to waste of radio resources. For this reason, there is a strong demand for less waste of power consumption and radio resources while using both of a radio voice communication system and an LTE system.